docs: archive historical planning documents and fix references

Move self-described historical/upstream docs to docs/archive/:
- agent-runbook-guide.md
- lan-core-switch-upgrade-plan.md
- lan-rb5009-upgrade.md
- se5420-review-claim-verification-2026-08.md

Update archive/README.md manifest and fix relative links in active docs
and archived docs. Update AGENTS.md docs/ layout description.

docs/lan-erx-se5420-network.md

@@ -307,7 +307,7 @@ VLAN10 / 升级 SSID / 客人 SSID **失败或未做，不否决**本次核心
 
 ## 13. 参考
 
-- 实施阶段与清单：[lan-core-switch-upgrade-plan.md](lan-core-switch-upgrade-plan.md)
+- 实施阶段与清单：[lan-core-switch-upgrade-plan.md](archive/lan-core-switch-upgrade-plan.md)
 - 现网地图：[lan-overview.md](lan-overview.md)
 - ER-X：[edgerouter-x-configuration.md](edgerouter-x-configuration.md)、[hosts/gw.md](../hosts/gw.md)
 - UniFi / VLAN10 前置：[unifi-network.md](unifi-network.md)